1. Field of the Invention
This invention relates to a dielectric ceramic composition for high frequencies. More particularly, the present invention relates to an improved dielectric ceramic composition which possesses a high dielectric constant and a high Q value even in a high frequency region of microwaves and millimetric waves.
2. Description of the Prior Art
In a high frequency region, various dielectric ceramics are widely used for dielectric resonators, dielectric substrates for microwave integrated circuits, and the like because of their useful electrical properties. In recent years, there have been trends to use higher frequencies in microwave systems, and there is an increasing demand for dielectric ceramics with better electrical properties.
To this end a ceramic composition of a system TiO.sub.2 -ZrO.sub.2 -SnO.sub.2 -Zno-NiO has been developed as a dielectric material for high frequencies and is disclosed in Japanese publication No. 55-34526. However, a flow Q value at high frequenceis of this material makes its general use impossible. For example, the Q values of this material are 25000 at 1 GHz, 12000 at 4 GHz, and 7000 at 7 GHz. Thus, when this material is used for dielectric resonators in a high power circuit operated at a low frequency, e.g., at 800 MHz, it generates a large amount of heat, thus making its stable operation impossible. Also, the Q value of this material is too low to use it as dielectric resonators operated at a frequency of 4 GHz or 10 GHz.
In order to solve this problem, it has been proposed to replace ZnO or NiO with Ta.sub.2 O.sub.5 in the system TiO.sub.2 -ZrO.sub.2 -SnO.sub.2 -ZnO-NiO in Japanese patent laid-open Nos. Nos. 58-51406 and 58-217465. The former composition consists essentially of a main component composed of 22 to 43 weight percent (wt %) of TiO.sub.2, 38 to 58 wt % of ZrO.sub.2 and 9 to 26 wt % of SnO.sub.2, and additives of not more than 7 wt % of ZnO and not more than 5 wt % of Ta.sub.2 O.sub.5. The latter differs from the former in that the additives consists of not more than 10 wt % of NiO and not more than 5 wt % of Ta.sub.2 O.sub.5.
The replacement of ZnO or NiO in the TiO.sub.2 -ZrO.sub.2 SnO.sub.2 -ZnO-NiO system with Ta increases the Q value in some degree, but the maximum Q value of these materials is 8000 at 7GHz. In addition, there is still room for improvement in their sintering properties.